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A Decade of Research on the Environmental Impacts of Pulp and Paper Mill Effluents in Canada (1992-2002)

4.5 Conclusions

Laboratory tests have been used to answer many of the regulatory and research questions that have arisen related to the effects of pulp mill effluents on aquatic biota. Acute, short-term, laboratory tests have also been used to assess final effluents and to track changes in effluent quality over time. Short exposures of fish and invertebrates have clearly shown the improvement in final effluent quality through the installation of secondary treatment of effluent.

In an effort to predict impacts on wild fish, laboratory bioassays have been developed to examine the responses seen in fish captured in pulp mill receiving environments: MFO induction and steroid depression. Short-term exposures of laboratory fish to pulp mill effluents have been used to identify pulp mill-related chemicals that induce MFO and suppress steroids.

Sublethal tests were developed to mimic the responses seen in wild fish captured from PME receiving environments. These tests were used to assess rapid responses of fish, such as hepatic MFO induction, and steroid depression. Methods for steroids assessment improved, and techniques to measure short-term responses of fish gonadal steroid production were developed.

These short-term bioassays have enabled biological responses, such as MFO induction or steroid depression, to be traced to specific chemicals and components of pulp mill processes (such as plant compounds, and black liquors). Assessment of MFO induction and steroid reduction caused by PME has enabled comparison of types of mill processes, wood furnish, and effluent treatment. It appears that MFO induction and steroid depression are found at a variety of mills types, with and without chlorine bleaching, in hardwood and softwood pulping facilities, and before and after effluent treatment.

New genomic techniques have contributed to the ability of laboratory bioassays to detect changes associated with exposure to PMEs. The development of chromosomal markers to determine the genetic sex of some fish species has led to some novel findings of feminization of male fish after exposure to high concentrations of BKME. The pattern of gene expression in fish affected by PME is different from the gene expression pattern in fish exposed to estradiol. Improvements in these genetic technologies, and expansion of genetic markers for smaller, shorter-lived, laboratory test species, may lead to the development of shorter bioassays that are linked to reproductive effects.

Longer-term, lifecycle laboratory tests and mesocosm exposures have been used to assess the effects of chronic exposure to pulp mill effluents. These studies have demonstrated PME effects on growth enhancement of invertebrates and fish, liver enlargement, and decreases in gonad size and fecundity in several fish species. Laboratory tests have assessed pulp mill waste streams, and provided information to isolate and treat selected pulp mill wastewaters.

Several early full lifecycle reproductive tests showed that PME could alter reproduction in fish. As these lifecycle tests were lengthy and expensive to conduct, the focus then shifted to developing predictive shorter tests that could be used to screen effluents for their potential to cause long-term reproductive effects in fish. At the present time we are addressing this last issue: The development of cost- and time-effective laboratory assays that will predict reproductive changes in wild fish exposed to PMEs.

Long-term laboratory exposures of fish to pulp mill effluents have examined reproductive parameters (egg production, time to first spawn). Effects on growth, maturation and reproduction have been seen in final effluents from a variety of pulp mills, employing different pulping processes, and different bleaching chemicals. The most sensitive endpoint in these long term fish exposures is reproduction. This endpoint is biologically meaningful, but determining thresholds for effects requires lengthy and expensive tests. As the laboratory tests move forward into the next decade, attention will focus on the reproductive endpoints, and the possibility of shortening the bioassays while still maintaining sensitivity and biological relevance.